This invention relates to amperometric measurement of a chemical in a flowing process stream. More particularly, this invention relates to an amperometric analysis method and system having process stream flow rate and temperature compensation. Advantageously, the system electrodes may be both ultrasonically cleaned and cleaned by electrochemical anodic pulse to provide a more reliable indication of stream concentration.
Amperometric methods of analysis to produce a current proportional to the concentration of the material measured have been known and applied in various industrial processes. See, for example, Smith, D. E. and Zimmerli, F. H., "Electrochemical Methods Of Process Analysis," ISA (1972), pp. 153-160. The amperometric measurement technique is based on the electrochemical oxidation or reduction of certain chemical species at the surface of suitable electrodes immersed in the solutions under test. The oxidation or reduction reaction involves the transfer of electrons between the chemical species and the electrode, thereby inducing a current flow. By selection of the appropriate applied potential, the particular species to be measured can often be selected.
The limiting current, that is, the maximum current for the reaction at the applied potential selected for the amperometric measurement for the component of interest, is linearly related to the concentration at fixed fluid velocity and temperature. Empirical studies demonstrate that the limiting current shows a temperature sensitivity of about +1.5 percent per degree centigrade and a linear relationship to approximately the cube root of hydrodynamic flow velocity.
While amperometry can be a sensitive and selective method for the direct measurement of "electroactive" materials, its use as such has been limited, finding primary application in amperometric titrations where amperometry is used indirectly to detect the appearance or disappearance of an electroactive substance which signals a titration endpoint.
The limited utility of amperometric analyzers as reliable direct chemical sensors can be attributed to several factors. Since the output current is a function of fluid flow, this has necessitated controlling the process flow, an often difficult task where it is desired to measure a process stream in as representative a fashion as possible. Also of significance is the loss of sensitivity of the instrument with time as the electrode becomes coated with insulating films. Since the measurement signal is derived from the flux of electrons across the electrode-solution interface energy barrier, any coating or deposit there can affect the measurement either by changing the flux by physical blockage of the electrode area or by synergistic chemical and physical effects on the active surface sites.
A prior art attempt at overcoming the latter aforementioned problem involves cleaning the surfaces of electrodes within a sensing cell by the continuous action of plastic balls pushed by a synchronous motor driven rotating striker. The action of the balls against the electrodes is said to maintain a constant level of cleanliness, thus eliminating drift while a thermistor may compensate for temperature variation in the sample. While such an amperometric analyzer is suggested, by the manufacturer, for continuous analysis of free or total chlorine residual in water, nevertheless, this approach involves the use of moving components and the mechanical difficulties of breakdown and maintenance associated therewith and still fails to achieve maximum reliability.